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ls120
2009-Nov-26, 03:02 AM
Background: As I understand it, in the 1990’s two independent teams of astronomers determined that the expansion rate of the universe appears to be accelerating, and the Hubble Constant is apparently not really “constant”, but increases with distance. The Hubble Constant is the ratio of expansion rate divided by distance and would be truly constant if the universe were expanding uniformly as many astronomers had expected.
To calculate the Hubble Constant for very distant galaxies the astronomer teams measured the redshift of type 1a supernova to determine the expansion rate of the universe, and they used supernova brightness to determine distance. They found that distant supernova were dimmer, therefore further away, than predicted by the measured redshift and the accepted value for the Hubble Constant. Since the distant supernova were further away than predicted, both astronomer teams concluded that the expansion rate of the universe must be accelerating.
Question: Could there be a different interpretation of the data as follows: Since the Hubble Constant is rate over distance (R/D), and the measured distance is greater, the value of the ratio must be smaller than expected. If that interpretation is true, the Hubble Constant is decreasing with distance, and the universe is decelerating as many predicted. Gravity is working as it should. There is no need for strange ideas like “dark energy” to explain an accelerating universe, and astronomers and physicists would be happy about that.

George
2009-Nov-27, 09:07 PM
Welcome aboard... No sense starting here with a goose egg with such a good question. :)


To calculate the Hubble Constant for very distant galaxies the astronomer teams measured the redshift of type 1a supernova to determine the expansion rate of the universe, and they used supernova brightness to determine distance. They found that distant supernova were dimmer, therefore further away, than predicted by the measured redshift and the accepted value for the Hubble Constant. Since the distant supernova were further away than predicted, both astronomer teams concluded that the expansion rate of the universe must be accelerating. I think that is right. The brigtness difference between an omega of 1 and an omega of 0 is about 25% for a z = 0.5. It was in the 1980s that the ability to obtain quality measurements of both redshifts and brightness for redshifts of about 0.5 prompted the search.


Question: Could there be a different interpretation of the data as follows: Since the Hubble Constant is rate over distance (R/D), and the measured distance is greater, the value of the ratio must be smaller than expected. If that interpretation is true, the Hubble Constant is decreasing with distance, and the universe is decelerating as many predicted. The change found demonstrates only acceleration, not deceleration. Assuming a continuous deceleration, however, presents an age paradox for the universe since the universe would be about 9 billion years old, which is younger than old stars.


Gravity is working as it should. There is no need for strange ideas like “dark energy” to explain an accelerating universe, and astronomers and physicists would be happy about that.I think they would be happy. :) But the evidence indicates acceleration. The diminished brightness of Type 1a supernovae reveals their additional distance. The time dilation of their brightness curve may be even a stronger argument for acceleration.

forrest noble
2009-Nov-28, 07:11 AM
ls120,


The Hubble Constant is the ratio of expansion rate divided by distance and would be truly constant if the universe were expanding uniformly as many astronomers had expected.


To calculate the Hubble Constant for very distant galaxies the astronomer teams measured the redshift of type 1a supernova to determine the expansion rate of the universe, and they used supernova brightness to determine distance. They found that distant supernova were dimmer, therefore further away, than predicted by the measured redshift and the accepted value for the Hubble Constant. Since the distant supernova were further away than predicted, both astronomer teams concluded that the expansion rate of the universe must be accelerating.


Question: Could there be a different interpretation of the data as follows: Since the Hubble Constant is rate over distance (R/D), and the measured distance is greater, the value of the ratio must be smaller than expected. If that interpretation is true, the Hubble Constant is decreasing with distance, and the universe is decelerating as many predicted. Gravity is working as it should. There is no need for strange ideas like “dark energy” to explain an accelerating universe, and astronomers and physicists would be happy about that.

(bold added)

As you indicated in your intro the Hubble Constant is the supposed constant rate of the expansion of the universe as expressed as a recession velocity. If this recession velocity (Hubble Constant or rate of expansion) is decreasing with distance as we look back in time, then it is increasing as we go forward in time. A continual increase in the rate of expansion as one goes forward in time is called accelerated expansion and is what has been claimed as the basis for the Dark Energy hypothesis.

It has been observed that for redshifts less than about .55, type 1a supernovae appear to be farther away than their calculated distance would indicate based upon their brightness. For those having redshifts greater than .65 they appear to be closer than there calculated distances would indicate based upon their brightness. At a redshift of about .6 their redshift and calculated distance seem to match based upon their observed brightness. http://www.astro.ucla.edu/~wright/sne_cosmology.html

Distance is calculated by the Hubble formula as D= v/ HO.
'v' , the recession velocity, is determined by the Hubble factor ß time 'c' (the speed of light).
ß (the Hubble factor to change redshift into distance) is calculated by using the observed redshift 'z', where
ß= ((z + 1)^2 -1)) / ((z + 1)^2 +1)). If this formulation for ß is a little bit off (needs tweaking) then the entire dark energy explanation could disappear.

The question might be what is simpler, that the universe expanded very rapidly, Inflation, then continuously slowed down up until about 5 billion years ago at which time it began to accelerate again. Or, that the Hubble distance formula (Hubble parameter as well as intrinsic brightness) might need adjustment. If this were the case then we could be talking about a maximum distance disparity of up to 25% farther away for the closest type 1a supernova distances and also at the farthest distances near the edge of the observable universe would be up to 25% closer. Those at a redshift of about .6 would accordingly be calculated correctly since at that distance their brightnesses seem to match calculations. This of course is not the only possibility other than accelerated expansion to explain what has been observed (see above link).

Sam5
2009-Nov-28, 06:01 PM
Background: As I understand it......


They found that distant supernova were dimmer, therefore further away, than predicted by the measured redshift and the accepted value for the Hubble Constant.




Maybe someone can explain something to me.

If it is the redshift that gave them the original “distance” estimation, and then the “faintness” of the star tells them the star is “farther away”, and if the star really is “farther away”, then why wouldn’t the redshift indicate that, since the amount of redshift is supposed to indicate the amount of distance? Seems to me that what they have is a discrepancy between the “redshift” distance and the “faintness” distance.

Cougar
2009-Nov-28, 07:11 PM
...in the 1990’s two independent teams of astronomers determined that... the Hubble Constant is apparently not really “constant”, but increases with distance.

Well, it's not a matter of the Hubble constant increasing with distance. The apparent recession velocity of distant objects increases with distance. For every megaparsec something is away from us, add another 72 km/sec to its apparent recession velocity. That's the Hubble "constant": around 72 km/sec per megaparsec.

I don't know that the Hubble constant was ever really considered to be constant throughout the age of the universe. It is the figure we get today from our vantage point. Prior to 1998, most astronomers figured the expansion rate, and hence the Hubble 'constant', must be slowing down due to the gravitational effect of all the mass in the universe, so the independent findings in 1998 that the rate was accelerating was a bit of a surprise. As Goldsmith put it....



"...astronomers thus found themselves astounded, if not totally floored, by what supernovae revealed... They have, however, leaped from the carpet, dusted themselves off, and proceeded to investigate the universe."

:)


...Since the Hubble Constant is rate over distance (R/D), and the measured distance is greater, the value of the ratio must be smaller than expected.

Again, Goldsmith:



"The cosmological constant's claim to a nonzero value fundamentally rests on the finding that distant Type Ia supernovae reach maximum brightnesses approximately 25 percent fainter than the peak brightnesses they would attain in a universe with a cosmological constant equal to zero."

forrest noble
2009-Nov-28, 08:21 PM
Sam5,


Maybe someone can explain something to me.

If it is the redshift that gave them the original “distance” estimation, and then the “faintness” of the star tells them the star is “farther away”, and if the star really is “farther away”, then why wouldn’t the redshift indicate that, since the amount of redshift is supposed to indicate the amount of distance?
On your second quote:

Seems to me that what they have is a discrepancy between the “redshift” distance and the “faintness” distance.

You are correct. What you call "faintness" distance is formally called "luminosity distance" and is calculated from the inverse square law of light. Redshift distance is calculated from the Hubble formula based upon the observed redshift. There is a discrepancy between the two.

I presume you are asking about type 1a supernova observations? If so then:

The problem is that these supernovae are believed to be standard candles. We believe we understand the mechanics of these explosions very well, which involve binary stars whereby one star explodes.

Based upon our understandings, their observed brightness does not match their calculated distances using the Hubble formula which is based upon their observed redshift which is transformed to a distance scaler by the Hubble factor ß.

As to why observations do not match calculations is a matter of speculation. The most prominent hypothesis is that the expansion of the universe is presently accelerating, if so, since the time of the explosion the EM radiation has had a longer distance to travel (is farther away now) and therefore appears dimmer. This is the basis of the dark energy, and accelerated expansion hypotheses. Other possibilities including possibly inaccurate formulations are discussed in posting #3 and the included link.

Hope this answers your question.

Sam5
2009-Nov-28, 09:10 PM
As to why observations do not match calculations is a matter of speculation. The most prominent hypothesis is that the expansion of the universe is presently accelerating, if so, since the time of the explosion the EM radiation has had a longer distance to travel (is farther away now) and therefore appears dimmer. This is the basis of the dark energy, and accelerated expansion hypotheses. Other possibilities including possibly inaccurate formulations are discussed in posting #3 and the included link.

Hope this answers your question.


Yes, thanks very much. Why don’t all the papers just say that and make it easy for everyone else to understand? :)

I might have another question later, after I think about your answer for a while.

Cougar
2009-Nov-28, 11:23 PM
Why don’t all the papers just say that and make it easy for everyone else to understand? :)

It may be that "is a matter of speculation" is a bit exaggerated. :confused: Obviously it's not a matter of random speculation. These observations tightly constrain any attempted explanation. That's not to say the best explanation we have today is set in stone or anything.

mugaliens
2009-Nov-29, 06:29 AM
Assuming a continuous deceleration, however, presents an age paradox for the universe since the universe would be about 9 billion years old, which is younger than old stars.

I would think "continuous" would be an erroneous assumption. And isn't the age of the "old stars" based on the assumption that the Hubble constant is indeed constant? Do we have any evidence to support that it would be the same today as it was 7 billion years ago? Why would it be the same in a younger, significantly different universe, with mass distributions more dense than they are today?

I'm not proposing anything - just raising some questions I haven't seen answered, yet.

Cougar
2009-Nov-29, 03:03 PM
And isn't the age of the "old stars" based on the assumption that the Hubble constant is indeed constant?

I don't think so....



Currently the best estimate for the age of the oldest stars is based on the absolute magnitude of the main-sequence turn-off in globular clusters... Independent ages of old objects can be obtained from nucleochronology... and from comparison of white dwarf cooling sequences with lower luminosity limits of white dwarf samples. -- more (http://www.rssd.esa.int/SA-general/Projects/GAIA_files/LATEX2HTML/node74.html)

George
2009-Nov-29, 09:31 PM
I would think "continuous" would be an erroneous assumption. Yes, and I think I worded that sentence poorly, but I was heading out the door at the time. It looks as if I was suggesting it as a posibility rather than showing why deceleration is a bigger problem than some might think.


Do we have any evidence to support that it would be the same today as it was 7 billion years ago? Yes. That is only a redshift of a little less than 1 and greater redshift data is available. [Of course, a little acceleration is what they found for the more recent history, and a little deceleration for the earlier period.]


Why would it be the same in a younger, significantly different universe, with mass distributions more dense than they are today? Good point, and it would explain the early deceleration period. [I believe that is the reason given.]


I'm not proposing anything - just raising some questions I haven't seen answered, yet. I'm no authority in such things but I'm getting better with y'all's help. [I fear I'm decelerating, especially noticeable after the Thanksgiving meal. ;)]